V, please stop! I did that up until the age of 14 and it takes years and years to get out of. Apart from harming the actual nail and cuiticle, you're ingesting all the crap you found in the street and it looks just awful, especially when they're painted. One of my nails is still damaged, I just can't grow it. Have you thought about stopping? You can get thin sheets of acrylic stuff that protects your nail as it grows, you won't bite that (only sometimes...)